2if 251 



found. Presumably an initial Jab of high tension such as cc in Figure lU 

 would have little effect on the gage. The observed tensions would represent, 

 therefore, merely the reflection of the tail of the incident wave from the 

 bottom of the cavitated region, as is stated in the report. 



The value of the breaking pressure may be inferred most easily from 

 the minimum depth at which the reflected tension appears in full strength, 

 indicating no cavitation. One of the observations mentioned points toward a 

 relatively high value of p^. A charge of 2 ^/h pounds of guncotton 6o feet 

 below the surface gave a maximum pressure of 9^0 pounds per square inch on a 

 gage placed 15 feet away and on the same level. Without cavitation, there- 

 fore, the maximum reflected tension should be about 91 x 15/120 =115 pounds; 

 but only 15 pounds was observed. Yet the maximum pressure at the surface 

 Would be only 9^0 x 1 5/60 = 230 pounds per square inch. If the gage was cap- 

 able of measuring tensions effectively, the conclusion is Justified that in 

 this case the water must have cavitated at a tension scarcely exceeding 200 

 pounds. 



It must be recognized, however, that cavitation at the gage might 

 alter the conclusions materially. If cavitation over the gage occurs at 

 higher pressures than it does in the water itself, then the tensions indi- 

 cated by the gage set only a lower limit to the magnitude of the tension oc- 

 curring in the water itself. The piezoelectric observations would be 

 consistent with the assumption that no cavitation at all occurs in the midst 

 of the sea. 



A few remarks may be added concerning the similarity laws for sur- 

 face phenomena. On page 9 it has been seen that the change to model scale, 

 as it is commonly made in dealing with underwater explosions, is possible 

 only so long as gravity effects can be neglected. In this change all linear 

 dimensions and all times are changed in one and the same ratio* the pressures 

 and velocities at corresponding points remain unchanged. It follows that the 

 effects of air pressure upon surface phenomena will be relatively the same 

 upon all scales. Insofar as these phenomena are Influenced by gravity, how- 

 ever, similar motions on different scales are impossible. Similar motions 

 would be possible only if the strength of gravity were changed in inverse 

 ratio to the linear dimensions, so as to preserve the value of the quantity 

 gL/v^ or, since v^ is unchanged, of gL itself; L is here any convenient 

 linear dimension and v is the particle velocity. Small-scale phenomena thus 

 correspond to large-scale ones occurring in a proportionately weaker gravi- 

 tational field. 



This conclusion is surprising, for it appears to mean that spray 

 should be thrown to the same height by charges of all sizes. This would be 



